1. Field of the Invention
This invention relates to ferromagnetic iron oxide powder for magnetic recording, specifically, to ferromagnetic iron oxide powder having a high coercive force and improved stability to pressure and heat.
2. Description of the Prior Art
Ferromagnetic iron oxides for magnetic recording are required to have a high coercive force and exhibit superior orientability.
One effective means of increasing the coercive force of ferromagnetic iron oxide powder involves incorporating cobalt ions in the iron oxide to form a solid solution, and various methods of this type are suggested, for example, in U.S. Pat. Nos. 3,117,933 and 3,671,435, Japanese patent application (Laid Open) No. 101,599/73, and Japanese patent publication Nos. 6,538/66, 4,264/74, 27,719/66 (corresponding to U.S. Pat. No. 3,573,980), 15,759/73, 10,994/73 and 6,113/67. Magnetic recording media, such as magnetic tapes, made using the cobalt-containing iron oxides produced by such methods, however, have the defect that they are unstable to pressure and head, and the magnetic signals recorded become weak or are transferred to a great extent.
Other methods for increasing coercive force comprise adhering a layer of a cobalt compound or cobalt ferrite to the surface of magnetic iron oxide powder free of cobalt in the form of a solid solution, or growing such a layer on the surface thereof. Such methods are specifically disclosed, for example, in Japanese patent applications (Laid Open) No. 108,599/74, 37,667/75 and 37,668/75 and Japanese patent publication No. 49,475/74. Ferromagnetic iron oxide powders obtained by these methods are stable to pressure and heat and have improved print through characteristics as compared with those obtained by forming an iron oxide-cobalt solid solution per se.
The methods which comprise adhering a cobalt compound to the surface of a ferromagnetic iron oxide not containing cobalt can afford a product having a coercive force of about 500 to 600 Oe at the highest. On the other hand, by methods involving growing a cobalt ferrite layer, the coercive force of the resulting product is nearly proportional to the volume of the cobalt ferrite layer grown on the surface. The cobalt ferrite layer is a magnetic oxide layer in which cobalt is dissolved to form a solid solution, and is unstable to pressure and heat. This defect of the cobalt ferrite layer grown on the surface gradually shows itself, and this method, as a result cannot provide ferromagnetic iron oxide powder having a high coercive force and improved stability to pressure and heat.